1. Field of the Invention
The present invention relates to a private electronic exchange system and, in particular, to a technique for detecting call metering of call traffic of the central office line call of a key telephone subscriber in a key telephone system.
2. Description of the Related Art
Generally, call metering was detected according to the call traffic of a key telephone subscriber in a key telephone system. The call metering of the key telephone system was performed by a telephone exchange office. That is, the telephone exchange office checks the call traffic via a common telephone network (C.O), thereby imposing the call charge to the key telephone subscriber by the checked call traffic.
In earlier key telephone systems, a central processing unit controlled a switching circuit and various other elements to connect a key telephone subscriber with a central office line. The central processing unit in these earlier key telephone systems however could not detect metering pulses supplied from the telephone exchange office of the central office line. Hence, the key telephone system could not precisely determine the call charges. In view of this, only the call time of a telephone call could be displayed on a display unit of the key telephone subscriber. This resulted in the call time being prolonged and the call charges being higher than I believe to be necessary.
One exemplar of early systems, U.S. Pat. No. 672,546 of D. M. Bliss, entitled Telephone Metering System, clearly illustrates that the concept of having a telephone metering device at the subscriber end of a telephone line, the metering system receiving a signal from the central office for determining the charges, is in fact old. Similarly, U.S. Pat. No. 3,097,266 and U.S. Pat. No. 4,146,746 to Luscher and de Crepy et al., entitled Telephone Fee Counter and Device For Computing And Displaying The Price Of A Service, each also use metering systems for determining the call charges at the subscriber end of a telephone line based on monitoring charge pulses on the telephone line. U.S. Pat. No. 4,065,640 to Rouiller entitled Apparatus For Interconnecting A Telephone Line And A Portable Calculator also attempts to construct a telephone apparatus that uses charge pulses and other signals from a telephone line to determine the actual call changes at a subscriber end of a telephone line. Both U.S. Pat. Nos. 5,048,079 and 5,291,547 to Harrington et al. and Boatwright et al. entitled SMDR Translator and Call-Disosition Analyzer, each endeavor to provide telephone apparatus for PBX systems that are able to use the signals on the trunk lines to determine the call charging information for various subscribers.
I have found that the art, as evidenced by these references, lacks an effective technique for detecting call charges for telephone calls via a key telephone system.